1. Field of the invention
The present invention relates to the processing of steel parts, and more specifically to a thermal carburizing process, that is, the introduction of carbon into the surface of the parts to improve their hardness. The present invention more specifically relates to carburizing equipment under vacuum or under a low gas pressure (lower than atmospheric pressure).
2. Discussion of the Related Art
In a low-pressure carburizing process parts to be processed are submitted, in an air-tight chamber, to an alternation of steps of enrichment in the presence of a low-pressure carburizing gas and of steps of diffusion under vacuum or under a low-pressure neutral atmosphere. The respective durations of the enrichment and diffusion steps as well as their number especially depend on the desired carbon concentration and case depth in the parts, and such processes are well known in the art. An example of a low-pressure carburizing process is described in French patent application No 2,678,287 of the applicant. A carburizing process is a thermal processing at high temperature (generally in the range of 800xc2x0 C. to 1000xc2x0 C., or even more) and the heating as well as the maintaining at a homogeneous temperature of the parts in the diffusion and enrichment steps are a key point of carburizing processes.
The present invention also relates to carbonitridation, having, as only difference with respect to the carburization, the enrichment gas used, to which ammonia is generally added. The well known result thereof is the forming of nitride (instead of carbide for the carburization) at the part surface. It should thus be understood that the following explanations in relation with carburization also applies to carbonitridation.
Generally, carburizing chambers define volumes of one or several cubic meters which are heated and maintained at the carburizing temperature by electrical heating means. In practice, electrical resistors in the form of bars, which are distributed at the periphery of the carburizing volume, that is, around the carburizing chamber, according to the desired thermal distribution and to the thermal bridges linked to the chamber structure, are used.
It would be desirable to have another carburizing chamber heating energy instead of electricity.
The first energy that comes to mind is gas, which is a xe2x80x9ccleanxe2x80x9d and inexpensive energy. However, the use of gas for heating carburizing chambers raises a great number of problems which have led, up to now, to preferring electrical heating, in particular for low-pressure equipment.
A first problem has to do with the very structure of gas burners, which must heat up the internal space of the chamber without introducing any gas combustion smoke therein. In this regard, the necessary length of the burners due to the large dimensions of the carburizing chambers is a critical point in terms of heat distribution in the chamber.
A gas burner system which would be of proper use corresponds, for example, to the burner system described in U.S. Pat. No. 4,894,006. This burner system is formed of a tight external envelope and of a central furnace tube delimiting a combustion chamber. Such a system uses a recirculation of the burned gases and enables the gases to come out at high speed. This burner system may be associated with an internal tube of the type described in U.S. Pat. No. 4,850,334. The respective contents of the above-mentioned publications being incorporated by reference.
Another problem linked to the use of gas tubes for the heating of a carburizing chamber, in particular a low-pressure chamber, has to do with the bulk of these tubes, which is substantially greater than the bulk of electrical resistive bars. This bulk goes against an adequate distribution of gas tubes in the periphery of the useful volume of the chamber to obtain a homogeneous distribution of the temperature.
Another problem is the necessary regulation of the thermal power of the used heating source. Indeed, the batch of parts to be carburized must first be brought to a high carburizing temperature. Then, this temperature must be homogeneously maintained during the steps linked to the carburization. In an electrical system, the temperature regulation is particularly easy to perform by modulation of the current in the heating elements. Such a solution cannot be transposed to gas burners
An object of the present invention is to provide a gas-heated carburization cell that overcomes the above-mentioned disadvantages.
Another object of the present invention is to provide a solution which is compatible with the current distribution of the heating means at the periphery of a carburization cell.
Another object of the present invention is to provide a modular carburization equipment that takes advantage of the use of gas as a heating power source.
To achieve these objects, the present invention provides a low-pressure cell for thermally processing steel parts, including heating means formed of several radiant gas tubes distributed around a useful volume of a tight chamber; and control means provided with at least one mode of pulse regulation of the heating means.
According to an embodiment of the present invention, the control means are adapted to controlling the heating means according to two operating phases, respectively of full power preheating and of temperature hold in pulse regulation.
According to an embodiment of the present invention, the control means are adapted to modifying the gas flow between two levels, respectively a maximum level for the preheating and an intermediary level for the pulse regulation.
According to an embodiment of the present invention, all radiant gas tubes are individually controlled or controlled by groups.
According to an embodiment of the present invention, the control means include a programmable state machine for individualizing control signals to be sent to the different tubes.
The present invention also provides an equipment for thermally processing steel parts under low pressure, including several processing cells connected to a common tight chamber provided with handling means for transferring a load from one cell to another, at least one cell being of the above-mentioned type.
According to an embodiment of the present invention, at least one cell is dedicated to the preheating of a load to be carburized, and at least one cell is a carburization cell.
According to an embodiment of the present invention, the carburization cell is provided with gas heating means adapted to being controlled in pulse regulation mode.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.